General purpose rubbers are employed in the manufacture of a wide variety of rubber articles including automotive tires. The term "general purpose rubber" as used in the specification and claims means the highly unsaturated rubbers of commerce illustrative of which are styrene butadiene rubber (SBR), polyisoprene, polybutadiene, natural rubber, etc. While these rubbers exhibit excellent physical properties in their vulcanized state, they are subject to attack by elemental oxygen, especially ozone. The resistance to oxidation and oxidative degeneration may be improved by the addition of an antioxidant or antiozonant at a concomitant increased cost in the rubber product.
It is well known in the art to improve the ozone resistance of such general purpose rubbers by blending the general purpose rubber (GPR) with an EPDM terpolymer. See, for example, U.S. Pat. No. 3,492,371, incorporated herein by reference.
It has been found that the EPDM rubber may be made more compatible with the GPR by brominating the EPDM. See, for example, U.S. Pat. No. 3,524,826, incorporated herein by reference. The disclosures of that patent teach that a requirement for operability of the invention is that the EPDM contain at least one wt. % bromine. The blends so formed have improved ozone resistance in addition to outstanding physical properties.
Heretofore, although compositions of GPR and EPDM demonstrated improved ozone resistance, they were lacking in flex properties and therefore had limited application in tire flexing components.